1. Field of the Invention
The present invention relates to an improvement of a pulse Doppler ultrasonic diagnostic system.
2. Description of the Prior Art
A pulse Doppler ultrasonic diagnostic system is intended to obtain a kinetic direction and a velocity of a ultrasonic scatterer by extracting only ultrasonic scattering signals from a region of concern within a subject for examination by processing Doppler shift components. In order to fetch the ultrasonic signals from the region, signals from an interval .DELTA.x between a depth x and a depth x+.DELTA.x have to be accumulated in any form and sample-held. The accumulation of signals involves the use of a method of integrating reflection signals with transmit ultrasonic waves serving as single pulse waves for a period equivalent to a sample volume length .DELTA.y by use of an interval integrator, or alternatively a method of sample-holding the reflection signal with the transmit ultrasonic waves serving as burst waves, the number of waves corresponds to the interval .DELTA. after a given time has passed, with a wave transmitting time being defined as an origin. Mean power of the transmit ultrasonic waves is limited to a constant value or under in terms of security in connection with influences of the ultrasonic waves on a human body. Under such a condition that the mean power is restricted to the constant value, however, it is impossible to invariably obtain best Doppler signals with respect to an arbitrary sample volume length simply by employing any one of the above-mentioned methods.